Burglar alarm system



P12525, 1936. R. v L. HARTLEY BURGLAR ALARM SYSTEM Filed Feb. 25, 1952 A TTORNE Y Patented Tres. 25, i936 cuirs STATES when l BURGLAR ALARM SYSTEM Ralph V. L. Hartley, South Orange, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporatiomof New York Application February 25, 1932, Serial N0. 595,046 Y.

l Claim.

105 'vision in an enclosure o'a continuously operated v vibrating element, the amplitude or phase or both of which will change whenever a change occurs in the acoustical impedance of the enclosure.

Another feature of this invention resides in the 15 :means provided for preventing false operation of the signal device when any change in the amplitude or phase of the'vibrating element is caused by a change fin the operating current-of thevibrating element. 2o*

protective means in a vault and an enclosure'for the vault and aseparate signal device for each enclosure. I

'Inthe drawing: Y 25:: fFig. -1 shows a schematic diagram of 'si complete system for 'detecting entrance to a vault; and

--Fig.2showsa schematicdiagram of 'the modiii'cation in whichv'a vault' is protected byan enclosure and 'a-'separate detecting system is provided to'pick up sound vibrations caused by anyone entering the vault 'or attempting to break through the wall. To Vfully, protect the contents of a vault and make sure thatl any unlawful entry to the "interior of the vault will be detected, all of thel above mentioned protective devices have to be ineluded' in the protective means employed. `A protective systemincluding all of the above mentionedfprotectivev means is quite expensive. If the locks, door switches and the networks in the wallrstructures are eliminated' and reliance is placedvon the soundpick-up device there is al,V Ways the possibility of some one gaining-entrance 55 to the' interior of the. vault without vproducing A modification of this invention includes like sound waves suiiicient to-cause operation of the sound pick-up device.

In the present invention a vibratory element is located withinthe vault or other .enclosure oi the contentsto be protected. The vibratory element 5 is Icontinuously operated"to 7 7 ce sta'dln 'g :milled n esire. AAny opening made e-e closure walls such, for instance, as opening of a door'or boring through the walls of the enclosure will result -in-va change in the acoustic impedance-of the interior of the enclosure.. The

amplitudeor hase or both amplitude and phase l nu l 'ofi/an ralarnigdevi Y: Y p

Fora complete description of this invention reference willnow be had to the drawing Yin which like -parts in the gures bear like numerals. -In Figui., -i Iis a vault. comprising airont wall 2, arear'wall 3, the side walls 4 4, a-oor and ceiling, not shown,-and a door 5 locatedmerely for the purposeof illustration in the front wall 2. The rear Awall 3 is apertured at 6 sui'licient to receive a high efficiency loudspeaker 'l which in this instance isshown asa moving coil type loudspeaker. The loudspeaker has an electromagnet 8 including a coil 9 supplied-vrith current from a battery I, a vibrating element II and aV moving coil i2 to operate the vibrating element-I I. The 30; vibrating element I I is directed through the aperture 6 :toward the "Interior of the vault I. The

moving coil i2 'is included in a Wheatstone bridge circuit I3 and forms one arm of the bridge circuit, the remaining three arms comprising a fixed resistance I4, a iixed resistance I5, a variable resistance R-IG, in series with a variable inductance L-IG, respectively. Bridge I3 is supplied with alternating current from an alternating current vgenerator I'I at the input points I8 and i9. The circuit including the source of alterna'ting current i1, the switch 31 and the conductors leading to the input points I8 and Y.I9 ofthe bridge i3 forms the input circuit for the bridge i3. Connected to the neutral points 2U and 2| of the bridge I3 are conductors 22 and 23 respectively which lead to a detector 24 Acapable- 'of detecting unbalancing effects in the bridge I3 and `which may,for instance, be a detector of the vacuum tube type. The conductors 22 and 23 and the detector 24 form an output circuit for the bridge I3. The output of the detector 24 is led by means of conductors Y25 and 26 to an amplifier 21, the output end of which is connected by conductors 28 and 29 to the operating coils ofc;

PATENT ortica i 65 ing current generator Il'.

a polarized relay 3D. The polarized relay 30 is maintained polarized by current from a battery SI supplied by Way of the conductors 32 and 33,

closed switch 34 and the variable resistance 35 5 and may be regulated as to its sensitiveness by adjustment of the variable resistance 35. The polarized relay 30 controls an alarm 36 which may be located at a central station.

To set the system in condition for operation the door 5 of the vault I is closed. The switches 34, 31 and 38 are closed and the generator Il is set in operation to supply current through the bridge I3 to the coil I2. The variable resistance R-IG and the variable inductance L--IG in the bridge I3 and the variable resistance 35 in the circuit of the polarized relay 3D are then regulated until no signal is obtained from the alarm 36.

Assuming that the door 5 of the vault I is closed, the switch 3'! of the circuit including the generator Il is closed, the switch 38 in the magnetic circuit of the loudspeaker 'l is closed and that the variable resistance R--I6 and variable inductance L-IS in the bridge I3 have been adjusted so that the bridge I3 is in a perfectly balloudspeaker 'l to produce Within the vault I a finding Wave. The electrical impedance of the loudspeaker 7 will remain constant so long as the acoustical impedance of the interior of the vault I remains unchanged. Opening of the door `5V or creating any opening in the Wall of the vault f or even changing the position of an object within the vault I will change the acoustical impedance withinthe enclosure and cause variation in the electrical impedance of the loudspeaker T.

A variation in the electrical impedance of the loudspeaker 'I will result in an unbalance of the 25 anced condition, the generator I'I will operate the bridge I3 and cause a current to now in the 4output circuit of the bridge I3. This current 40 will pass through the conductors 22 and 23 and 4.5 operated will move its armature 39 against back contact 40 and close the circuit of the alann 35.

Since it is highly desirable in the protection of bank vaults to have an alarm system that is Sulliciently sensitive to detect not only opening of the vault door but also the presence in a Wall of the vault of a comparatively small opening made by a person attempting to break through the wall of the vault, I have devised the system described to meet such conditions. It is also desirable in my vault alarm system to produce within the vault a standing wave of only comparatively low intensity in order not to attract attention to the fact that the vault is equipped with an alarm system. In the sysem described, the standing-wave produced within the vault may be of very low intensity. To produce the required standing wave of comparatively low intensity, the loud-speaking type receiver is operated on comparatively weak alternating current furnished bythe alternat- When the' bridge I3 becomes unbalanced, the current produced in the output circuit of the bridge I3 will naturally be comparatively weak. To detect the comparatively Weak current produced in the output circuit a A vacuum tube detector element may be included in the output circuit of the bridge I3. The comparatively weak current detected by the detector element 24 may be sufficiently amplied by means of asuitable amplifier 21 to operate a relay. The

relay controls an alarm device 36 in a local alarm circuit. I have found that a polarized relay is particularly suitable for the purpose of controlling the local alarm circuit since a polarized relay may be adjusted to a very sensitive characteristic and the current detected by the detector element 24 need only be amplified by the amplifier 21 to a comparatively small extent to operate the polarized relay.

The generator Il should be a reliable source of current supply and the loudspeaker be very efficient at the particular frequency employed. However, since the loudspeaker 1 is included in the bridge I3 and the bridge I3 is normally in a balanced condition any slight variation inthe supply to the loudspeaker l from the generator I'I will not result in a false signal.

The loudspeaker should be very eicient at the particular frequency employed and should be operated to produce Within the enclosure a standing wave of comparatively low frequency. The electrical impedance of the loudspeaker will depend to a considerable extent on the reaction of the air within the vault on the wave producing surface of the loudspeaker. The dimensions of the vault should also be considered in regard to the frequency employed.l I have fgund that by using a high eiciency loudspeaker of .tleugnoving coil type and with a stretched diaphragm byv operating this loudspeaker at a frequency of about 200 cycles in a room about 20 by 30 feet with a 12 foot ceiling that an appreciable change in the electrical impedance of the loudspeaker occurs when a door olf the room is opened. The change in the electrical impedance of the loudspeaker due to the change in the reaction of the air within the room caused by the opening of the door is sufficient to cause an unbalance in the bridge circuit. The electric current which iiows in the 'conjugate arm of the bridge when this unbalance occurs may be amplifiedv to control a signal. I have also found that signals are produced with the same apparatus when objects are moved about in the room.

In Fig. 2 the vault I is equipped with the same protective system shown in Fig. 1 and in addition is located in an enclosure 4I comprising a front wall 42, a rear wall 43, the side walls 44-44, a floor and ceiling not shown, and a door 45. A protective system similar to the system described for the valut I is provided to detect any opening of the door 45 or break in the walls of the enclosure 4I. The high efficiency loudspeaker I is located in an aperture 46 of one oi' the side Walls 44. The magnetizing coil 9' of the loudspeaker l' is supplied with current from battery I0. A'lhe vibratory element II'` is operated by the moving coil I2. The moving coil I2' forms one arm of the Wheatstone bridge I3', the remaining amis comprising a iixed resistance I4'.,.a iixed resistance I5', and a variable resistance R-IB' in series with the variable inductance L-IB', respectively. The bridge I3 is supplied with current from an alternating current gen-- erator I1 at the points I8' and i9'. Connected to the output points 20 and 2l' of the bridge I3 are conductors 22 and 23', respectively, which lead to the detector 24. Signals detected by the detector 24' are amplified by the ampller 21 and pass to the polarized relay 30 which is supplied with polarizing current from the battery 3 I by way of conductors 32 and 33 closed switch 34 and the variable resistance 35. The polarized relay 30' controls an alarm 36 located at a central station. Y

When it is desired to detect any opening of the door 45 or breaking through of any of the walls of the enclosure 4l, the door i5 is closed and a. standing wave is produced within the enclosure 4l by means of the loudspeaker 'I'. if the door '45 is opened or any break is made in the Walls of the enclosure 4l or an object is moved about within the enclosure 4l the acoustic impedance Within the enclosure 4l changes and causes a change in the electrical impedance of the loudspeaker A change in the electrical impedance of the loudspeaker 7' causes an unbalance in the bridge I3 and the transmitting of a signal to the detector 24. The signal transmitted is ampliied and passed on to the polarized relay 30. The polarized relay 30 operates and moves its armature 3S against back contact 40 and closes the circuit of the alarm 36'. The system shown in Fig. 2 provides not only for detecting a forced or unauthorized entrance to the vault I but also a forced or unauthorized entrance to the enclosure 4| in which the vault i is located.

It is obvious that certain changes may be made in the number and character or arrangement of the apparatus parts shown and described and that the loudspeaker elements may be located entirely `Within the respective enclosures and only Wires pass through the walls instead of having the loudspeaker project through the Wall Without departing from the spirit of this invention.

What is claimed is:

In a burglar alarm system for a vault, in combination, a loud-speaking type receiver having a diaphragm Within the vault, a balanced electrical bridge system including in one of its arms said loud-speaking type receiver, a source of alternating current connected to input points of said balanced electrical bridge System to operate said loud-speaking type receiver to produce sustained air vibrations Within the vault, a relay in circuit connection with output points of said balanced electrical bridge system, a local circuit controlled by said relay and an alarm device in said local circuit, said alarm device being operated when a change occurs in the acoustic impedance of the.

interior of the vault sufficient to cause unbalance of said balanced electrical bridge system.

RALPH V. L. HAR'ILEY. 

